This invention relates to an eye tracking sensor, and more particularly to an eye tracker using electrically switchable gratings.
Eye tracking is important in Head Mounted Displays (HMDs) because it can extend the pilot's ability to designate targets well beyond the head mobility limits. Eye tracking technology based on projecting IR light into the users eye and utilizing the primary Purkinje (corneal) reflection and the pupil-masked retina reflection have been around since the 1980's. The method tracks the relative motion of these images in order to establish a vector characterizing the point of regard. Most eye trackers have employed flat beam splitters in front of the users' eyes and relatively large optics to image the reflections onto a sensor (generally a CCD or CMOS camera).
There is much prior art in the patent and scientific literature including the following United States filings:                1. United Stated Patent Application Publication No. US2011019874(A1) by Levola et al entitled DEVICE AND METHOD FOR DETERMINING GAZE DIRECTION;        2. U.S. Pat. No. 5,410,376 by Comsweet entitled EYE TRACKING METHOD AND APPARATUS;        3. U.S. Pat. No. 3,804,496 by Crane et al entitled TWO DIMENSIONAL EYE TRACKER AND METHOD FOR TRACKING AN EYE TWO DIMENSIONAL EYE TRACKER AND METHOD FOR TRACKING AN EYE;        4. U.S. Pat. No. 4,852,988 by Velez et al entitled VISOR AND CAMERA PROVIDING A PARALLAX-FREE FIELD-OF-VIEW IMAGE FOR A HEAD-MOUNTED EYE MOVEMENT MEASUREMENT SYSTEM;        5. U.S. Pat. No. 7,542,210 by Chirieleison entitled EYE TRACKING HEAD MOUNTED DISPLAY;        6. United Stated Patent Application Publication No. US 2002/0167462 A1 by Lewis entitled PERSONAL DISPLAY WITH VISION TRACKING; and        7. U.S. Pat. No. 4,028,725 by Lewis entitled HIGH RESOLUTION VISION SYSTEM.        
The exit pupil of these trackers is generally limited by either the size of the beamsplitter or the first lens of the imaging optics. In order to maximize the exit pupil, the imaging optics are positioned close to the beamsplitter, and represent a vision obscuration and a safety hazard. Another known limitation with eye trackers is the field of view, which is generally limited by the illumination scheme in combination with the geometry of the reflected images off the cornea. The cornea is an aspheric shape of smaller radius that the eye-ball. The cornea reflection tracks fairly well with angular motion until the reflected image falls off the edge of the cornea and onto the sclera. The need for beam splitters and refractive lenses in conventional eye trackers results in a bulky component that is difficult to integrate into a (HMD). The present invention addresses the need for a slim, wide field of view, large exit pupil, high-transparency eye tracker for HMDs.
The inventors have found the diffractive optical elements offer a route to providing compact, transparent, wide field of view eye trackers. On important class of diffractive optical elements is based on Switchable Bragg Gratings (SBGs). SBGs are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the film. A volume phase grating is then recorded by illuminating the liquid material (often referred to as the syrup) with two mutually coherent laser beams, which interfere to form a slanted fringe grating structure. During the recording process, the monomers polymerize and the mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the film. When an electric field is applied to the grating via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range. The device exhibits near 100% efficiency with no voltage applied and essentially zero efficiency with a sufficiently high voltage applied. In certain types of HPDLC devices magnetic fields may be used to control the LC orientation. In certain types of HPDLC phase separation of the LC material from the polymer may be accomplished to such a degree that no discernible droplet structure results.
SBGs may be used to provide transmission or reflection gratings for free space applications. SBGs may be implemented as waveguide devices in which the HPDLC forms either the waveguide core or an evanescently coupled layer in proximity to the waveguide. In one particular configuration to be referred to here as Substrate Guided Optics (SGO) the parallel glass plates used to form the HPDLC cell provide a total internal reflection (TIR) light guiding structure. Light is “coupled” out of the SBG when the switchable grating diffracts the light at an angle beyond the TIR condition. SGOs are currently of interest in a range of display and sensor applications. Although much of the earlier work on HPDLC has been directed at reflection holograms transmission devices are proving to be much more versatile as optical system building blocks.
Typically, the HPDLC used in SBGs comprise liquid crystal (LC), monomers, photoinitiator dyes, and coinitiators. The mixture frequently includes a surfactant. The patent and scientific literature contains many examples of material systems and processes that may be used to fabricate SBGs. Two fundamental patents are: U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka et al. both filings describe monomer and liquid crystal material combinations suitable for fabricating SBG devices.
One of the known attributes of transmission SBGs is that the LC molecules tend to align normal to the grating fringe planes. The effect of the LC molecule alignment is that transmission SBGs efficiently diffract P polarized tight (ie light with the polarization vector in the plane of incidence) but have nearly zero diffraction efficiency for S polarized tight (ie light with the polarization vector normal to the plane of incidence. Transmission SBGs may not be used at near-grazing incidence as the diffraction efficiency of any grating for P polarization falls to zero when the included angle between the incident and reflected light is small.
There is a requirement for a compact, lightweight eye tracker with a large field of view, and a high degree of transparency to external light.